Over time, two types of networks were developed in most countries. One is a public telecommunications network (e.g. a telephone network, ISDN-network) and the other a considerable number of private and special networks. In the following, the private network concept is used for this type of telecommunications network.
In most cases, a private network consists of a private branch exchange, to which several telephone sets are connected, and which has available a connection to the public network. Such a network is e.g. often found in companies or authorities.
If a company has several different locations, each of these locations requires its own private branch exchange. These private branch exchanges are often interconnected by leased dedicated public lines. Calls within the company require dialing the private branch exchange of the calling partner through these dedicated public lines.